1. Field of the Invention
The present invention relates to a nitride semiconductor Light Emitting Diode (LED), and more particularly, to a flip chip nitride semiconductor LED of excellent electric characteristics and luminance.
2. Description of the Related Art
Recently, a nitride semiconductor LED as an optical device for generating blue or green wavelength light is made from semiconductor material expressed by an equation of AlxInyGa(1−x−y)N (wherein 0≦x≦1, 0≦y≦1 and 0≦x+y≦1). Considering lattice match, nitride semiconductor crystals are grown on a substrate such as a sapphire substrate that is used for the growth of nitride single crystals. Since the sapphire substrate is electrically insulated, p- and n-electrodes are formed on the same side of a final nitride semiconductor LED.
According to structural characteristics as above, nitride semiconductor LEDs have been positively developed into specific geometries adequate to flip chip structures. FIG. 1 illustrates a flip chip light emitting device having a conventional nitride semiconductor LED mounted thereon.
A flip chip light emitting device 20 shown in FIG. 1 includes a nitride semiconductor LED 10 mounted on a supporting substrate 21. The nitride semiconductor LED 10 includes a sapphire substrate 11 and an n-doped nitride semiconductor layer 12, an active layer 13 and a p-doped nitride semiconductor layer 14 formed in their order on the sapphire substrate 11. The nitride semiconductor LED 10 may be mounted on the supporting substrate 21 by welding electrodes 19a and 19b with lead patterns 22a and 22b via conductive bumps 24a and 24b, respectively. In the flip chip light emitting device 20 of this structure, the sapphire substrate 11 of the LED 10 may be used as a light emitting plane since it is transparent.
Each electrode, in particular, the p-electrode of the flip chip nitride semiconductor LED is required to have high reflectivity for reflecting emission light from the active layer 13 toward the light emitting plane while forming an Ohmic contact with the p-doped nitride semiconductor layer 14 as shown in FIG. 1. Therefore, as shown in FIG. 1, a p-electrode structure may include a high reflectivity Ohmic contact layer 15 formed on the p-doped nitride semiconductor layer 14 and a metal barrier layer 16 for preventing the diffusion of components of the Ohmic contact layer 15.
However, since the nitride semiconductor LED 10 shown in FIG. 1 has a planar electrode structure, and in particular, the p-electrode side Ohmic contact layer 15 has a lower specific resistance (for example 5 to 10 mΩ/cm2) lower than that of the p-doped nitride semiconductor layer 14, the nitride semiconductor LED 10 of this type has current crowding in which a major portion of current flowing along the Ohmic contact layer 15 is concentrated in a narrow part A adjacent to the n-electrode as indicated with an arrow.
Such current crowding increases forward voltage while lowering the luminous efficiency of an active layer portion relatively remote from the n-electrode to degrade luminance properties. Further, the current concentrated part A generates a large quantity of heat thereby remarkably degrading the reliability of the LED.